Bearing assemblies used in a majority of low-friction rotational couplings are subject to wear, and can be damaged by use when worn. Typically bearing assemblies are defined by a raceway between two opposing races, in which bearing elements are retained. It is also common to have a cage which constrains the bearing elements to motion within a range to ensure a distribution of the bearing elements within the raceway, resulting in a balanced distribution of stresses imparted on the bearing assembly.
Skidding, the gross sliding of a bearing element bearing surface relative to one of the races is a principal indicator (and cause) of wear. Skidding is generally a high-speed phenomenon caused by a difference between inner and outer race-bearing element loading (mainly caused by the centrifugal force of the bearing element). Increasing applied load to the bearing can decrease skidding, but will tend to reduce fatigue endurance. So a compromise between the degree of skidding allowed and bearing endurance must generally be accepted, and lubrication regimes are chosen with the degree of skidding in mind.
Skidding results in surface shear stresses of significant magnitudes. If a lubricant film generated by the relative motion of the bearing element within the raceway is insufficient to completely separate the surfaces, surface damage known as “smearing” will occur. Smearing is a severe type of wear characterized by metal tightly bonded to a race and/or the bearing element caused by transference. Smearing causes roughness in the contact areas which is detrimental to any bearing assembly. An amount of skidding is to be controlled in any application since at the very least it results in increased friction and heat generation even if smearing does not occur.
Skidding is a particular problem in cylindrical bearing element bearings used to support shafts in high speed and/or high load applications. These bearings, which are used principally for localization of spinning parts, are very lightly loaded while operating at high speeds making them very susceptible to skidding.
In some applications high radial stresses are applied to bearing assemblies, especially when the bearing assemblies support shafts that rotate at high rates. In some applications nominal stresses are applied, but vibrations, imbalance of the bearing assembly, or failure caused by worn or otherwise damaged bearing assemblies, can result in catastrophic failure of critical systems. While backup systems and other failsafe measures are built into may critical systems, the use of bearing assemblies still requires preventative maintenance programs. Typically, to reduce a likelihood of failure, bearing elements are replaced after a number of operating hours according to a Diagnostic, Prognostic and Health Management program, or the like. The reliance on a number of operating hours is not an ideal solution because of a high cost of precision bearings and the shortening of their duty life, costs of down time of the equipment, possible absence of a backup for a critical system while one system is taken off-line or costs of multiple backup systems to ensure that there is a backup, etc. Consequently, in choosing the number of operating hours (or corresponding measure of amount of use) before replacement, a trade off is made between reducing a probability that the bearing will undergo a failure, and the costs of replacing the bearing.
A need for in situ sensors has therefore been acknowledged, and a number of these systems have been developed. A majority of bearing monitoring systems known in the art appear to use vibration sensing equipment to detect the onset of failure.
While vibration sensing equipment may provide adequate notice for some applications, in others the bearing assembly has suffered irreversible damage by the time the failure of the system is detected. Once vibrations are manifest, the damage sustained may extend beyond the bearing elements to the cage, and to the opposing races necessitating replacement of larger pieces. Furthermore, with vibration analysis, it may be difficult to detect failures and problematic operation for equipment in large, interconnected, complex machinery, as identification of which parts caused which vibration may be difficult.
Further still applicant has found that vibration analysis does not work on turbines of jet engines, for example. These high-efficiency rotational couplers are very quiet, and consequently, even in isolation, vibration analysis does not provide desirable information.
In particular, as is well known in the art, skidding of bearing elements within bearing assemblies is of particular interest for determining how long a bearing assembly should be used in the given mode of operation. To this end, it is highly desirable to be able to compute a ball pass frequency to determine a rate of revolution of the bearing elements within the raceway.
It has recently been suggested to provide sensors within bearing assemblies, either within bearing elements themselves, or within the cage. For example, it has been suggested to provide sensors (i.e. eddy current displacement gauges) within a center of a cylindrical ball bearing, as taught by Kazao et al. in Japanese Patent Abstract application number 57204590 (publication number 59097316 A).
PCT application WO 2006/083736 to Varonis teaches an antifriction bearing having a sensing unit for sensing a condition of the bearing, wherein the outer race has a power transmitting coil and a receiver and the cage has a power receiving coil and a transmitter for sending a signal of the sensed condition of the bearing to the receiver in the outer race.
While there may be applications for which these systems are suited, Applicant has found that small variations in mass to a cage causes an imbalance of the whole bearing assembly that is unacceptable for bearing assemblies that operate at high revolution rates. For example, an experiment where small notches were removed in a cage for a bearing assembly for radial load designed to operate up to 35,000 rpm failed at about 20,000 rpm in one experiment. Accordingly in some applications (including most all high speed applications), cage mounted sensors could not be implemented without some very accurate weight distribution control that may not be feasible or desired at a given cost point, if at all possible.
Furthermore the expense of multiple receiving and transmitting coils embedded in the cage and an outer race, according to the teachings of Varonis, and equipment for communicating the signal to a processor increase costs of parts and engineering design requirements of the overall system.
Other systems have been designed that detect vibration directly at an outer race surface to monitor and analyze bearing conditions. Strain gages were initially used by Shapiro of the Franklin Institute. Later the strain gages were replaced by non-contact fiber optic techniques by Philips et al. of the Naval Research and Development Center (U.S. Pat. No. 4,196,629). Bentley Nevada of Minden Nevada has published on the Internet, a paper outlining a system for monitoring and analyzing bearing conditions that summarizes the above evolution and replaces the fiber optic techniques with an eddy current proximity transducer.
In these systems, detection equipment is arranged on a wall of the bearing assembly, abutting a piece providing the outer race, which is consequently a bearing wall. If the piece is too thick, or irregularly shaped, it can be difficult to correctly associate deflections with events within the bearing assembly. However, it is common practice to provide replaceable bearing assemblies which have relatively thin radial walls that transmit these deflections, and mechanical deflections are most noticeable at this location. A probe according to the system described by Bentley Nevada is directed radially from an axis of rotation of the bearing assembly on the radial, bearing wall. Thus the eddy current probe is aligned with the principle stresses borne by the bearing assembly.
Unfortunately, equipment in use provides a housing surrounding the radial wall of bearing assemblies, and this housing is important for providing uniform structural support for the bearing assembly. To provide a probe with access to the piece providing the outer race through the attendant equipment, an opening is needed in the housing. It is, however, not desirable to provide an opening in the housing as this presents a structural weakness in the housing. Such a structural weakness may actually decrease a service life of the bearing assembly. Accordingly knowledge of properties of the bearing assembly tested in a modified housing may not be an accurate predictor of the bearing assembly in use in equipment with the unmodified housing. Furthermore, it is not generally possible to test equipment in situ with this method, unless the housing is altered to provide a structural weakness, or a modified housing is provided. A time needed for replacing the housing with an altered housing for in situ testing and returning the housing for continued normal use may make this method less attractive. Furthermore, any inaccuracy of the results of testing of a bearing assembly, or any imperfections in the bearing assembly caused by operation within the altered housing may be of concern.
There therefore remains a need in the art for a technique for monitoring a bearing assembly.